Our principle goal is to determine the physiological mechanisms which regulate suppressor T cell activity. To do this, we separate suppressor T cells from other T cells with the use of monoclonal antibodies against differentiation antigens. We then separate other subsets of cells and mix them with the suppressor T cells. After incubation of the two populations of cells we remove one or the other with the appropriate antiserum against unique differentiation antigens and ask (1) how the suppressor T cells have affected the function of the other cell set with which they were mixed and (2) the reciprocal. Besides mixing cells and assaying T cell activity, we are attempting to isolate and characterize the biologically active mediators that these T cells use in communicating with one another.